iOS笔记之单例模式的理解

@interface SingleDemo : NSObject
+ (instancetype)sharedSingleObject;
@end

@implementation SingleDemo
+ (instancetype)sharedSingleObject {
return [[self alloc] init];
}

+ (instancetype)allocWithZone:(struct _NSZone *)zone {

static SingleDemo *demoObject;

static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
//该代码块中的内容仅会被加载一次
demoObject = [super allocWithZone:zone];
});

return demoObject;
}
@end